The present invention relates to a radioactive dust monitor adopting an electrostatic collection type electric dust collection system, and more particularly to a radioactive dust monitor which includes a dust collecting electrode serving also as a radiation detection surface, an ionization line extending in air in a spaced relation with the dust collecting electrode and a radiation detection part and is capable of continuously measuring a rays radiated from dust collected on the dust collecting electrode. Since this apparatus is small and light-weighted and ensures a silent operation, it is useful for the measurement of the concentration of radioactive material in air in the vicinity of workers or in an operating environment as well as for the continuous measurement of the concentration of radioactive material in the inside of a glove box.
In nuclear fuel material handling facilities, for example, when unsealed radioactive material is handled, a radioactive dust monitor for measuring the concentration of radioactive material in air in an operating environment within a controlled area is used, in order to enable the early finding of the leakage of radioactive material from a glove box or the like and to reduce the internal exposure of workers at the time of contamination.
A conventional radioactive dust monitor adopts an air suction system, and is formed, for example, of a combination of a suction pump, a dust collection part collecting dust in air sucked by the suction pump and a radiation detection part detecting radioactive rays. That is, air is sucked with the use of the suction pump and the dust floating in air is collected by a filter paper in the dust collection part. Then, the radioactive material concentration of the collected dust is measured using the radiation detection part incorporated in the monitor.
However, in such a conventional radioactive dust monitor which employs the suction pump, there has been a drawback that the suction pump is heavy and large-sized and hence, the monitor lacks in portability. Further, since the conventional monitor has a movable portion, it has a structural problem in terms of the continuous running ability, noise and the like. Still further, it is difficult to avoid the discharge of sampled air. Due to these reasons, an area in which the conventional monitor can be installed is limited and, particularly, it is not suitable for use in the state that it is installed in a glove box.
It is an object of the present invention to provide a radioactive dust monitor which exhibits an excellent portability since the monitor can be miniaturized and light-weighted, ensures the silent operation, enables the stable continuous running for a long period since the monitor has no movable portion, and generates no exhaust, whereby an area in which the monitor can be installed can be enlarged.
It is another object of the present invention to provide a radioactive dust monitor which can strengthen the administration of the radioactive material concentration in air in a working environment so that the high measurement accuracy can be achieved.
The present invention is directed to a radioactive dust monitor comprising a dust collecting electrode serving also as a radiation detecting surface, an ionization line extending in air in a spaced relation with the dust collecting electrode and a radiation detection part. The radiation detection part being provided with a scintillator disposed close to the dust collecting electrode and a photoelectric conversion part converting light emitted from the scintillator into an electric signal.
In such a radioactive dust monitor having the structure described above, dust in air is collected by the dust collecting electrode due to a corona discharge which is generated by applying a negative high voltage to the ionization line and by applying a positive high voltage to the dust collecting electrode, and light emitted from the scintillator due to radioactive rays radiated from the collected radioactive dust is detected by the photoelectric conversion part.
The present invention is characterized by adopting an electrostatic collection type electric dust collection system in place of a conventional air suction pump system. As shown in FIG. 1, the electrostatic collection type electric dust collection is a system in which a negative high voltage is applied to an ionization line 10 and a positive high voltage is applied to a dust collecting electrode 12 to thereby generate a corona discharge by locally forming an area having a strong electric field, and thus dust 14 in air is ionized and the resulting ions are moved to the dust collecting electrode 12 due to the Coulomb force acting in the electrostatic field and the dust is collected on the front of the dust collecting electrode 12. In the radioactive dust monitor according to the present invention, this dust collecting electrode 12 also serves as a radiation detection surface so that the radioactive dust present in air is collected and is measured at the radiation detection part.
It is preferable to use a metal layer of a light shielding film as the dust collecting electrode. The light shielding film is disposed on a front surface of the scintillator to shield an external light. This light shielding film is to be constituted such that although the external light is shielded, the transmission of the radioactive rays (xcex1 rays) is allowed and is formed of a plastic film having a metal layer thereon, for example. The metal layer is used as the dust collecting electrode. In this case, it is preferable that the scintillator comprises a transparent substrate on a surface of which a scintillator layer is formed and the scintillator layer is disposed close to a back-surface side of the light shielding film. This is because, in the measurement of xcex1 rays, the distance between the scintillator and an object to be measured (radioactive dust) should be made as small as possible to obtain high efficient measurement.
The radiation detection part comprises a scintillator layer formed on a surface of a transparent substrate, a large number of optical fibers arranged in contact with each other and disposed at a back-surface of the transparent substrate and two photomultiplier tubes. The scintillator layer is disposed close to a back-surface side of the light shielding film, the optical fibers are alternately divided into two groups and the two photomultiplier tubes are respectively connected to the two groups of optical fibers. In such a radiation detection part having the structure described above, a current pulse simultaneously outputted from both of the two photomultiplier tubes is detected as a radiation signal and a current pulse outputted from only one of the two photomultiplier tube is eliminated as noise.